The present invention relates to a machine tool. More particularly, the invention relates to a clamping device for holding a tool holder securely to a hollow rotating spindle.
Tapered shank tool holders have a knob on a top end. A bit or other tool is fixedly held in the tool holder. The tool holder is releasably held within a tapered socket of a hollow rotating spindle driven by the machine tool.
The spindle has an axial channel open into the tapered socket. Collet fingers are captured within the channel. The collet fingers are actuated to move and turn within the axial channel to grip the tool holder knob. A draw bar slidable within the axial channel is engaged to the collet fingers. Depending on the axial position of the draw bar in the axial channel, the collet fingers are either in the knob-gripping clamped position or the knob-releasing unclamped position.
A plurality of springs, in the form of Bellville washers securely hold the tool holder in the spindle tapered socket by urging the draw bar to turn the collet fingers in the clamping position. The knob held by the collet fingers is pulled upward into the operating position, and the tapered shank is tightly fit into the tapered socket.
To release the tool holder from the clamped position in the spindle, pressurized hydraulic or pneumatic fluid is applied within a cylinder to act on a piston connected to the drawbar. The fluid forces the drawbar downward against the urging of the Bellville washers to unclamp the tool holder. When a new tool holder is to be installed, to return the collet to the clamping position, the fluid pressure is released. The Bellville washers snap the drawbar back to the clamping position to clamp the knob on the new tool holder.
U.S. Pat. No. 3,271,853 describes a tool holder having a head that is captured by two pivoting gripping members. The gripping members are pivoted about centers thereof by pins and are opened and closed by cam surfaces on an outside surface.
U.S. Pat. No. 3,791,257 describes jaw members that are pivoted open and closed. These jaw members engage a socketed portion of a tool holder.
U.S. Pat. Nos. 8,556,554; 8,403,338; 8,376,670; 8,066,456; 7,367,097; 7,284,938; 6,287,059; 6,234,731; 5,730,562 all describe tool holder head-engaging mechanisms. However, these engaging mechanisms are in the form of collets or chucks that use the vertical sliding of a drawbar to position collet portions beneath the spindle head of the tool holder.
The present inventor has recognized that collet designs can suffer the drawback that with enough force on the tool by the workpiece being machined, the tool holder can move in the tapered socket. To prevent this movement the spring force by the Bellville washers must be high. When the pressure is released from the pressurized fluid during a tool holder clamping operation, the Bellville washers tend to collide the tool holder into the tapered socket with an undesirable impact.
The present inventor has recognized that the spindle ball bearings are adversely affected by the impact caused by the engagement of the clamping mechanism on release of the air or hydraulic pressure. The drawbar is raised rapidly by the Bellville washers and the reaction force or impact between the tool holder and the spindle is transferred through the ball bearings into the machine tool. The release and clamping of tool holders can occur many times per day, possibly a thousand times, and the repetitive impact on the ball bearings can shorten the life of the ball bearings.
The present inventor has recognized that it would be advantageous to provide a clamping arrangement for a tool holder that resists movement between the tool holder with the spindle at even great forces from the workpiece. The present inventor has recognized that it would be advantageous to provide a clamping arrangement for a tool holder that was effective to reduce needed spring force to preserve spindle bearings, which are expensive.